This invention relates to photoactive materials which are useful in imaging systems. More particularly, the invention is concerned with materials which are capable of generating free bases on interaction with radiation.
Photogeneration of active species has proved to be extremely important in many areas and photolabile protecting groups have been used widely in organic and bio-organic synthesis. Photocurable polymers have found many applications in coating technology and photoresists are important in fabrication of microelectronic devices. Although some such applications involve very short wavelength ultraviolet irradiation or even electron beams, most commercial systems use ultraviolet or visible light sources because of their ease of use for low cost.
In lithography, compounds that produce radicals and/or acids on photolysis have been used extensively as photoinitiators in positive and negative working imaging systems. Thus, irradiation with light may be used to effect polymer formation, or polymer side-chain modification. In situ generation of acid is used as a means of inducing polymerisation of monomers or oligomers or to effect cross-linking, and both of these processes normally lead to less soluble materials. Despite the possibilities for application of base catalysts, the use of photogenerated bases in imaging systems has attracted little attention. In most photochemical reactions that liberate a base (usually an amine), it is trapped in solution in its largely neutral protonated form so that such processes are of little utility with systems that require base catalysts. Major exceptions lie in deep-UV irradiation of transition metal-amine complexes with negative photoresist systems, which photogenerate ammonia in a quantum efficient process and short wavelength irradiation of xcex1,xcex1-dimethyl-3,5-dimethoxybenzyloxy carboxamides, which yield amines. Thus, compared with acid-release systems, there are few general base-release agents useful for short- and long-wavelength lithographic applications.
The present invention seeks to provide organic photoprecursors of amines which may find application in lithographic printing plates or photoresist systems, and methods for the production of said photoprecursors.
Advantageously, said photoprecursors may be caused to photochemically generate free amines via interaction with long wavelength UV or visible radiation and may then serve to cause a change in the dissolution properties of a coating composition in developer solutions, thereby effecting differences in solubility properties in exposed and unexposed regions and facilitating image formation. Alternatively, changes in colour between irradiated and non-irradiated areas may be caused to occur with the incorporation of base-sensitive dyes into the coatings, thereby allowing for ready distinction between image and non-image areas.
The conception of generation of amines by irradiation of systems with light is well known from the synthesis of peptides or nucleotides, wherein photolabile protecting groups are used until the final steps of the synthesis in order to protect the amine. Cleavage of the masking groups then allows the amine to be regenerated, preferably by irradiation with visible or long-wavelength UV light, in a process having high quantum efficiency.
An especially suitable photolabile group for systems of this type is the 2-nitrobenzyl group. Particular attention has focused on 2-nitrobenzyl compounds, in which the benzylic group carries at least one xcex1-hydrogen atom; these derivatives are known to undergo photoinduced intermolecular oxygen transfer with reduction of the nitro group to a nitroso group and simultaneous oxidation of the benzylic side chain. This process is initiated by photoinduced hydrogen abstraction by an ortho-nitro group from the nearby benzylic carbon functionality, followed by transfer of a hydroxyl group from the nitro group back on to the carbon atom from which the hydrogen was abstracted.
Several derivatives of this type have been reported in the prior art. Photosensitive polymers which are used to form resist patterns, and which contain o-nitro benzyl oxy units are disclosed in unexamined Japanese Patent Specification No. 63-146032 and unexamined Japanese Patent Specification No. 63-247749 describes a range of o-nitro aryl oxy and o-nitro hetaryl oxy unit-containing polymers useful in the photosensitive layers of offset printing plates.
Subsequently, unexamined Japanese Patent Specifications Nos 03-131626 and 03-141357 have disclosed photosensitive amphipathic high molecular weight compounds which comprise polyesters or polyester acids or their esters which are prepared from tetracarboxylic acid dianhydrides and diols containing o-nitrobenzyl groups, the acid dianhydride optionally being reacted with an alcohol and converted to an acid halide prior to reaction with the diol. The compounds are utilised in light sensitive ultra-thin films.
More recently, improved quantum efficiency of photoreaction has been reported when employing dinitrobenzyl derivatives as precursors for photoreactive compounds as disclosed, for example, in U.S. Pat. Nos. 5,449,834 and 5,600,035. Thus, various 2,5- and, most preferably, 2,6-dinitro-benzyl derivatives have been used as monomers for the preparation of photosensitive polymers, having especially enhanced sensitivity at longer wavelengths. Particular use has been made of dichlorodinitro compounds and dinitrodiols in this regard, with 2,6-dinitro-4-methoxycarbonylbenzaldehyde having found particular application. Various polyurethanes, polysulphides, polyesters, and polyamines have been prepared from these precursors.
Unfortunately, however, whilst a high degree of photosensitivity is achieved with these products, their synthesis is not a trivial matter and the materials are only obtained with difficulty. Additionally, the compounds, like many polynitro derivatives, suffer from poor solubility in a range of common organic solvents, such as hydrocarbons, alcohols, ketones and the like.
It is, therefore, an object of the present invention to provide highly photosensitive compounds, capable of efficient photoreaction allowing for the production of basic materials during said photoreaction, wherein the photosensitive compounds and corresponding precursors may be simply and efficiently prepared by means of standard synthetic techniques.
It is a further object of the present invention to provide highly photosensitive compounds as hereinbefore described, having a high level of solubility in a range of common organic solvents.
The present inventors have found that the general principles discussed may be applied to urethanes having the general structural formula I. 
Thus, on irradiation, the compound I undergoes light induced internal rearrangement to produce nitroso derivative II and carbamic acid III. Due to the instability of the acid III. its formation is followed rapidly by spontaneous release of carbon dioxide to give the free amine IV. Hence, suitable urethanes I are photoprecursors of amines and may be used as photoactive compounds for photoimaging.
Furthermore, it has also been found that polymeric derivatives, including for example polycarbonates or polycarbamates which include the 2-nitrobenzyl functionality will undergo similar light-induced degradation. Thus, photolysis of a polycarbamate results not only in polymer degradation by cleavage at the carbamate linkages, but also in functional changes on the resulting fragments which contain terminal basic amino groups, whereas the original polymer is neutral. Therefore, such polymers may be used to provide a positive-working photoimaging system wherein the irradiated areas of a coating may be washed away with an aqueous acidic developer, thereby producing a three-dimensional relief image, for example via the degradation of a polymer V to a nitroso derivative VI and a diamine VII. It has also been found that these polymers are of value in the production of negative-working photoimaging systems, since the breakdown productsxe2x80x94containing basic amino groupsxe2x80x94which are present in the irradiated areas of the coating are insoluble in aqueous alkaline developers containing a small amount of solvent, whereas such developers cause dissolution of the unirradiated areas due to the presence of polymers which include urethane groups carrying acidic protons; such polymers thereby confer solubility in aqueous alkaline solutions, particularly in the presence of small amounts of solvents which enhance the solubility of the polymers. 
A first aspect of the present invention provides organic photoprecursors of amines for use in photosensitive imaging systems, said photoprecursors generating free amines on exposure to long wavelength UV or visible radiation and having high solubility in organic solvents, and said photoprecursors including photolabile 2-nitrobenzyl functional groups.
Said photoprecursors are especially useful in photosensitive systems used for the production of lithographic printing plates.
Preferably, said photoprecursors including photolabile 2-nitrobenzyl functional groups comprise 2-nitrobenzyl urethane derivatives of the formulae I or VIII or derived polyurethanes of the formulae V, IX and X. Derivatives I are prepared by employing monoisocyanates in the synthesis, whilst diisocyanates are used for the preparation of compounds, V, VIII, IX and X. 
Particularly favourable results have been achieved with 2-nitrobenzyl urethane derivatives wherein the benzyl group is substituted with an alkyl group, preferably with a methyl group. Optionally, the aryl ring may be further substituted, most preferably with methoxy groups, or other groups capable of producing bathochromic shifts in the wavelength of maximum absorption of the parent (unsubstituted) compound.
It has been found that optimum sensitivity for compounds which are intended for use in deep UV photolithography, at around 250 nm, is achieved when the 2-nitrobenzyl ring carries no further ring substituents. On the other hand, bathochromic shifts of the main absorption band of the order of 100 nm may be achieved by the introduction of electron donating substituents, preferably methoxy groups, into the 2-nitrobenzyl ring. Consequently, derivatives which are especially suitable for near UV/visible photolithographic applications may be obtained.
A second aspect of the present invention provides a method of preparing a lithographic printing plate comprising the steps of:
a) providing a lithographic printing plate precursor comprising a substitute and a photosensitive coating comprising an organic photoprecursor according to the first aspect of the invention;
b) imagewise exposing the printing plate precursor to radiation; and
c) developing the plate.
A lithographic printing plate precursor may be conveniently obtained by dissolving an organic photoprecursor according to the first aspect of the invention in any of a range of common organic solvents and coating the solution on to a suitable lithographic substrate. Potential coating solvents typically include aromatic hydrocarbons such as toluene, alcohols, for example ethanol or isoproponal, and ketones such as acetone or methyl ethyl ketone, or mixtures of any of these solvents.
Optionally, the photosensitive coating layer may also include additional lithographically useful materials, for example: support resins, preferably those containing groups capable of conferring solubility or swellability in aqueous solutions; colour change dyes, including especially pH sensitive colour change dyes; shading dyes; pigments; sensitisers; stabilisers; surfactants and other suitable materials.
The material used for the substrate depends upon the specific purpose for which the image is to be used and may be, for example, a metal, paper or plastics material. In the case where the image is to be used as a printing image, the substrate is preferably aluminium, most preferably electrochemically roughened aluminium which includes a surface layer of anodic aluminium oxide.
The resulting printing plate precursor is imagewise exposed to long wavelength UV or visible radiation, most conveniently by means of, for example, a Berkey-Ascor printing down frame. Consequent photodegradation of the precursor in the radiation struck areas allows for the generation of free amine in these areas, thereby rendering the exposed coating suitable in aqueous acidic media. These areas may then be developed away by application of a suitable acidic developer, leaving a positive image on the plate which may be used for the production of copies on a printing press.
Alternatively, as previously observed, it is possible to obtain a negative image by judicious selection of the developing solution to make appropriate use of the solubility differential which exists between the exposed and unexposed areas of the coating. Thus, by incorporation of a small amount of a suitable solvent, typically benzyl alcohol, in an aqueous alkaline solution containing an appropriate anionic surfactant, for example a sulphonated naphthalic acid derivative, a developer is obtained which may be used for the preparation of a printing plate from a photographic negative, the plate subsequently finding use for the production of copies on a printing press.
Such so called image-reversal techniques, whereby both positive- and negative-working images may be produced from the same printing plate precursor, are already known from the prior art and are disclosed, for example, in GB-B-2188448. However, the teachings of the earlier art inevitably require the use of additional steps, such as a blanket exposure of the plate or a further baking procedure, during the platemaking process. By application of the present invention such additional stages of platemaking can be eliminated thereby leading to savings in terms of both time and cost, with a consequent improvement in efficiency.
In terms of the press performance of lithographic printing plates including photoprecursors according to the first aspect of the present invention, it is found that enhanced performance, in terms of greater durability, is achieved by the use of polymeric derivatives as hereinbefore described. Particularly favourable results may be obtained by the use of polymeric photoprecursors in combination with support resins.
A further aspect of the present invention, provides a method of preparing organic photoprecursors of primary amines according to the first aspect of the invention, said method comprising the steps of:
(i) nitrating an aromatic aldehyde or ketone XI to provide the o-nitro derivative XII;
(ii) reducing said o-nitro derivative XII to the corresponding alcohol XIII; and
(iii) reacting said alcohol XIII with mono- or di-isocyanates to provide 2-nitrobenzyl urethane derivatives I (R2=H) or VIII. 
Preferably, said aromatic aldehyde or ketone comprises a ketone, preferably acetophenone or 3,4-dimethoxyacetophenone, said reduction is performed by treatment with sodium borohydride, and said mono or di-isocyanate comprises an optionally substituted alkyl or aryl mono- or di-isocyanate, preferably butyl isocyanate, hexane-6,6-diisocyanate, phenyl isocyanate, 4,4xe2x80x2-diisocyanatodiphenylmethane or 4-nitrophenylisocyanate.
The reaction of said alcohol with mono- or di-isocyanates is most advantageously carried out at elevated temperatures, preferably 30-150xc2x0 C., most preferably 50-120xc2x0 C., in an aromatic hydrocarbon solvent such as toluene, under an inert atmosphere. High yields are generally achieved.
Polymeric derivatives may be prepared by employing bisaldehydes or bisketones which, by reaction of the derived nitro-alcohols with diisocyanates, allow for the preparation of bits (2-nitrobenzyl urethane) derivatives which then serve as precursors to polymers. Alternatively, polymers may also be prepared from the reaction of diisocyanates with 2-nitroalcohols containing further active hydrogen-containing groups, such as hydroxy or mercapto groups, for example 5-hydroxy-2-nitrobenzyl alcohol.
Said 2-nitrobenzyl urethane derivatives contain N-substituted amino groups, which may be further substituted by alkylation of the amino group using an alkyl halide, preferably methyl iodide, in order to provide organic photoprecursors of secondary amines. Said alkylation is most conveniently carried out in an efficient polar solvent, such as dimethylformamide, in the presence of a reductive catalyst, typically sodium hydride.
A still further aspect of the present invention provides a method of preparing organic photoprecursors of secondary amines according to the first aspect of the invention, said method comprising the steps of:
(i) nitrating an aromatic aldehyde or ketone XI to provide the o-nitro derivative XII;
(ii) reducing said o-nitro derivative XII to the corresponding alcohol XIII;
(iii) reacting said alcohol XIII with carbonyl chloride to provide the corresponding chlorocarbonate XIV; and
(iv) condensing said chlorocarbonate XIV with a secondary amine. 
Preferably, said aromatic aldehyde or ketone comprises a ketone, preferably acetophenone or 3,4-dimethoxyacetophenone, said reduction is performed by treatment with sodium borohydride, and said secondary amine comprises an optionally substituted dialkylamine, preferably a lower dialkylamine such as diethylamine or disopropylamine.
The successful preparation of the chlorocarbonates requires the presence of a constant large excess of carbonyl chloride (phosgene) in order to suppress formation of the corresponding bis-carbonate derivative. Under such conditions, yields in excess of 90% are achievable.
The inventors have demonstrated the usefulness of the products of the present invention as photoprecursors for amines by studying their behaviour in solution and in solid film. Thus, samples of photoprecursors, dissolved in suitable solvents such as benzene, methanol or tetrahydrofuran (THF), and deaerated by the passage of a stream of nitrogen, were irradiated with lamps having a spectral output forming a continuum from the deep UV to the near IR region, and having strong emission lines at 365, 405 and 436 nm; Oriel 500W Hg(Xe) arc lamps were particularly suitable for this purpose. Analysis of the solution over time during irradiation, by means of thin layer chromatography and gas chromatography-mass spectrometry (GC/MS), allowed for the photolysis process to be followed in terms of decrease of photoprecursor and appearance and increase in quantity of new materials, including nitroso compound and, most particularly, amine. It was noted, however, that the use of THF as solvent resulted in the formation of pyrrole derivatives on irradiation due to reaction between the solvent and the liberated amines; this phenomenon was not observed when employing other solvents.
In a similar fashion, irradiation of a photoprecursor on a filter paper in the presence of pH sensitive dye or dye indicator, such as bromothymol blue (pH range 6.0-7.6) was achieved by pouring solutions of both materials on to the paper and irradiating with the aforesaid light source. A yellow to blue colour change was indicative of the release of a base. Alternatively, the same effect was achieved by using an anodised aluminium plate, rather than a filter paper, as the substrate.